Iodine test is a chemical test that can tell the difference between monosaccharides and polysaccharides like amylase, glycogen, and dextrin.
What is Iodine Test?
The iodine test is a test used to determine the presence of starch in a sample. Iodine is a chemical element that reacts with starch to form a complex called “iodine-starch complex” which appears as a blue-black color.
The procedure involves mixing a small amount of the sample with a few drops of iodine solution. If starch is present in the sample, the mixture will turn blue-black, indicating a positive test result. If the mixture remains yellow, then no starch is present, and the result is negative.
Aim of Iodine Test
Using an iodine test, one can detect starch in a given sample. The iodine test can discriminate between starch and monosaccharides, disaccharides, and other polysaccharides.
Principle of Iodine Test
In the iodine test, polyiodide ions form colorful adsorption complexes with helical chains of glucose residues of amylase (blue-black), dextrin (black), or glycogen (reddish-brown). Monosaccharides, disaccharides, and branching polysaccharides like cellulose are non-pigmented. Amylopectin is orange-yellow in colour.
The reagent utilized in the iodine test is Lugol’s iodine, an aqueous solution of elemental iodine and potassium iodide. Iodine by itself is not soluble in water. When potassium iodine is added, the iodine ion combines reversibly with iodine to form a triiodide ion, which then reacts with another iodine molecule to form a pentaiodide ion.
The ions iodide, triiodide, and pentaiodide are colorless, whereas the bench iodine solution is brown. The helix (coil or spring) structure of the glucose chain appears to be important in the Iodine test. Furthermore, the length of the glucose chains determines the colour produced. The resulting ions, triiodide and pentaiodide, are linear and slip within the helix structure.
Charge exchange between the helix and the polyiodide ions is thought to cause changes in the spacing of the energy levels, allowing the complex to absorb visible light and give it its colour. The colour intensity decreases as temperature and water-miscible organic compounds such as ethanol are added.
The blue amylase-iodine complex dissociates when heated, but reassembles when cooled because the helical shape is broken; as a result, amylose loses its ability to bind iodine and blue colour. Cooling restores the blue hue by restoring the helical structure as well as the iodine binding capacity.
Iodine Test Requirements
Reagent: Lugol’s iodine.
Supplies: Test Tubes, Test tube stand.
Equipment: Water bath, Vortex.
Note: Lugol’s iodine is made by mixing 5% elemental iodine with 10% potassium iodide to make Lugol’s iodine.
Procedure of Iodine Test
- 1 mL of the sample should be added to a clean, dry test tube.
- In a separate tube, regulate 1 mL of pure water.
- Combine a few drops of Lugol’s solution and mix them together.
- Examine the colour in the test tubes to determine its appearance.
- In a water bath, heat the test tubes until the colour fades.
- Set aside the test tubes to cool.
- Take note of the coloration of the test tubes.
Figure 1: Iodine test for starch diagram | Diagram by NotesHippo
Iodine Test Results and Analysis
The result of the iodine test is based on the color change of the mixture of the sample and the iodine solution. If the mixture turns blue-black, then starch is present in the sample. If the mixture remains yellow, then no starch is present.
It is important to note that the presence of other molecules such as glycogen, cellulose, certain amino acids or lipids can also form complex with iodine and give positive results, so the interpretation of the test should consider the context of the sample being tested.
Figure 2: Image Shows Iodine Test Results
Applications of Iodine Test
- This test is commonly used to test the purity of flour and other cereal products to ensure that they meet industry standards.
- Scientists use the iodine test to study the distribution of starch in different parts of a plant, such as in the leaves, stem, and roots.
- Researchers use the iodine test to measure the efficiency of enzymes that break down starch, such as amylases and glucanases.
- The iodine test is used in many industrial settings to test the quality of starch-based products such as paper, textiles, and adhesives.
Limitations of Iodine Test
The iodine test is a simple and inexpensive test that is widely used in many different applications, but it also has some limitations that should be considered when interpreting the results. Some of the limitations of the iodine test include:
- False negatives: The iodine test may not detect small amounts of starch or may give a false negative result if the starch is present in a degraded form, such as maltodextrin, that is not recognized by the iodine.
- Interference from other compounds: The iodine test may give false positive results if the solution being tested contains other compounds that react with iodine, such as certain types of sugars or reducing agents.
- Inability to distinguish between different types of starch: The iodine test can only detect the presence of starch and cannot distinguish between different types of starch, such as amylose and amylopectin.
- Light Sensitivity : Iodine test solution is sensitive to light, so the test should be performed in a dark or low light area or using opaque containers.
- Colorimetric interference : Other colors or substances could interfere with the color change of the test solution.
- Inability to quantitate the amount of starch present: The iodine test does not give a quantitative measure of the amount of starch present in a sample.
FAQs on Iodine to test for starch
The iodine test is a simple, cheap, and commonly used chemical test to find out if a solution contains starch.
Iodine is not typically used to test the pH of water. Instead, pH test strips or a pH meter is typically used to measure the acidity or basicity of water.
Iodine solution for starch test is made by mixing 5% elemental iodine with 10% potassium iodide to make Lugol’s iodine.
Iodine is widely used to detect starch due to the chemical reaction that happens between iodine and starch. When iodine interacts with starch, it generates a compound that has a characteristic blue-black hue.
The iodine ions in the iodine solution are capable of penetrating the starch granules and reacting with the starch molecules at the site of the double bonds to produce the iodine-starch complex. The iodine-starch complex absorbs light in the visible spectrum, giving it its distinctive blue-black hue.
In a clean glass container, mix together a few crystals of iodine and about 2 grams of potassium iodide (KI) in about 50 millilitres of distilled water. Stir the mixture until the iodine and potassium iodide are completely dissolved. The solution should be a dark brown color.